Sulfonic acid group- and/or carboxyl group-containing fluoropolymers were initially developed mainly for the purpose of using them as ion exchange membranes to be utilized in common salt electrolysis, among others. Conventionally, such membrane-like molded articles are produced by molding —SO2F group-containing fluoropolymers by extrusion molding, for instance, followed by hydrolysis.
Sulfonic acid group- or like acid group-containing fluoropolymers have recently attracted attention as materials of not only ion exchange membranes for common salt electrolysis but also electrolyte membranes for fuel cells and chemical sensors, and so forth.
A solution of a sulfonic acid group-containing fluoropolymer in a mixed solvent comprising an alcohol is known as a medium for immobilizing a catalyst on the electrolyte membrane surface in the manufacture of electrolyte membranes and the like (cf. e.g. Japanese Kokai Publication Hei-08-236122). However, this solution has a problem in that it covers active sites of the catalyst in the process of drying, for instance, and thus cause deteriorations in performance characteristics of fuel cells (cf. e.g. Makoto Uchida: “Element technologies of and design guidelines for gas diffusion electrodes for PEFC”, Denkikagaku oyobi Kogyobutsurikagaku (English title: Electrochemistry), published by The Electrochemical Society of Japan, 2002, vol. 70, No. 8, p. 639). This solution has a further problem from the environmental and/or operational viewpoint. Therefore, aqueous dispersions of sulfonic acid group-containing fluoropolymers have been demanded.
Aqueous dispersions of sulfonic acid group-containing fluoropolymers as such can also be used like solutions, hence can adequately be used in film/membrane formation by casting or in immersion, for instance. Thus, they have a wide range of application.
A method currently used in preparing aqueous dispersions of sulfonic acid group-containing fluoropolymers comprises subjecting membranous molded articles made from —SO2F group-containing fluoropolymer to alkali treatment and then to acid treatment to convert —SO2F groups to sulfonic acid groups and, further, treating the molded articles in a mixed solvent composed of water and a lower alcohol or in water at high temperature and high pressure conditions.
The —SO2F group-containing fluoropolymer so far used in the art in preparing membranous molded articles are produced mostly by solution polymerization to obtain pellets for use in extrusion molding and like methods of producing membranous molded articles.
Conceivable as a method of obtaining aqueous fluoropolymer dispersions as an alternative to solution polymerization is emulsion polymerization. Generally, the polymers in polymer latexes prepared by emulsion polymerization are recovered by adding an electrolyte to the latexes to cause coagulation of polymer particles. However, it is a problem that the essential auxiliary components, such as the emulsifier and electrolyte, remain in the polymers; it is thus difficult to obtain high-quality aqueous fluoropolymer dispersions. Emulsifiers, in particular, are difficult to remove, raising a problem in that the driers are rusted by gases generated in the step of drying the polymers obtained and/or the polymers are decomposed in the step of film/membrane formation to produce bubbles and/or become dark colored, for instance.
Known as a method of obtaining aqueous fluoropolymer dispersions without using any conventional emulsifier is the method which comprises polymerizing a fluoromonomer(s), such as tetrafluoroethylene or/and vinylidene fluoride, in the presence of a perfluorovinyl ether containing a sulfonic acid group or carboxyl group, which may be in the form of a salt (cf. e.g. Japanese Kokai Publication Sho-59-196308, Japanese Kokai Publication Sho-55-29519 and Japanese Kokai Publication Hei-08-67795). However, regarding this method, these publications have no description about the use of —SO2F group-containing fluorovinyl ether derivative in the step of subjecting the monomer(s) to polymerization.
Also known as a method of obtaining aqueous fluoropolymer dispersions is the method which comprises using a fluoromonomer having —SO3Na or the like in the step of polymerization to give sulfonic acid salt type fluoropolymers without using any conventional emulsifier (cf. e.g. Japanese Kokai Publication 2001-226436 and Japanese Kokai Publication 2001-226425). However, the publications cited have no description about the method of obtaining aqueous dispersions of sulfonic acid group-containing fluoropolymers.
As for the method of obtaining aqueous dispersions of sulfonic acid group-containing fluoropolymers, a method is known, among others, which comprises treating membranous molded articles prepared from —SO2F group-containing fluoropolymer with an alkali and then with an acid to convert —SO2F to the sulfonic acid group and dissolving the membranous molded articles in a mixed solvent composed of water and a lower alcohol or in water by treatment under high temperature and high pressure conditions or treating the membranous molded articles in a solvent essentially consisting of water with stirring under high temperature and high pressure conditions to give an aqueous dispersion of particles of 2 to 30 nm in size (cf. e.g. Japanese Kohyo Publication 2001-504872).
However, the method disclosed in Japanese Kohyo Publication 2001-504872 is inefficient since the fluoropolymer in liquid form as obtained by polymerization is once made into membranous molded articles and these are again made liquid. Further, it is a problem that a high-temperature and high-pressure treatment is required and, therefore, the corresponding reaction apparatus and energy are required.
Furthermore, the polymer particles obtained by the method disclosed in Japanese Kohyo Publication 2001-504872 are known to have a rod-like or thread-like shape such that the aspect ratio is generally 5 to 6 and the major axis length is about 11 nm. However, in the case of aqueous dispersions prepared by dispersing such rod-like or thread-like polymer particles, it is necessary to remove, by evaporation, a large amount of the dispersion medium in forming films/membranes by casting or impregnation, for instance. This is very inefficient, and it is difficult to produce thick films/membranes. A further problem is that cracks are readily formed in the step of drying.